This invention relates to a process for passivating and stripping semiconductor substrates.
Electrically conductive features comprising metal-containing compounds are formed on semiconductor substrates to electrically connect the devices formed on the substrate. Typically, the features are formed by the steps of (i) depositing continuous layers of metal-containing compounds and alloys on the substrate, (ii) overlaying a patterned, protective, resist material on the metal-containing layers, and (iii) etching the unprotected portions of the metal-containing layer to form features underlying the resist protected portions of the metal-containing layer. The metal-containing layers are deposited on the substrate using conventional chemical and physical vapor deposition techniques, and comprise metals conventionally used in the formation of integrated circuits, such as for example, aluminum, titanium and tungsten. A patterned resist layer, such as oxide hard mask or polymeric resist, is formed on the deposited metal-containing layers, using conventional photoresist and photolithographic techniques. After the oxide hard mask or patterned resist layer is formed, conventional reactive ion etching processes using halogen-containing etchants, such as for example. Cl.sub.2, BCl.sub.3, CCl.sub.4, SiCl.sub.4, CF.sub.4 CF.sub.4, NF.sub.3, SF.sub.6, and mixtures thereof, are used to etch the metal-containing layers, as generally described in S. Wolf and R. N. Tauber, Silicon Processing for the VLSI Era, vol. I, Chap. 16: Dry Etching for VLSI, Lattice Press, Sunset Beach, Calif. (1986), which is incorporated herein by reference.
The reactive ion etching processes leave (i) corrosive etchant byproducts, (ii) remnant resist, and (iii) sidewall deposits on the sidewalls of the etched features. The etchant byproducts result from adsorption of the halogen-containing etchants on the etched features. Remnant resist are those residual resist portions that are not etched by the etchant gases, and which remain on the substrate after the etching process. The sidewall deposits are formed during the etching process, by reaction of the etchant gases, the metal-containing layers, and the resist; and condensation of the reaction byproducts on the sidewalls of the features.
The remnant resist, etchant byproducts, and sidewall deposits on the substrate are removed by stripping, passivating, and wet chemical etching methods. Conventional stripping methods which use a plasma of oxygen, and an oxygen activating gas such as CF.sub.4 or N.sub.2, can be used to strip polymeric remnant resist from the substrate. These conventional stripping techniques are sometimes ineffective at stripping all the resist from the substrate, particularly when the polymeric resist is hardened by exposure to a plasma. For oxide hard mask resists, the oxide layer can be stripped or can be left on the substrate. Typically, remnant oxide hard mask is left on the substrate and a dielectric layer is deposited on the remnant oxide in a subsequent process step.
Passivating and inhibition techniques are used to reduce post-etch corrosion problems. After etching, the substrate is exposed to ambient moisture in the atmosphere, and the etchant byproducts on the substrate react with absorbed moisture to form corrosive species which corrode the etched features on the substrate. For example, when chlorine-containing gases are used to etch aluminum features on the substrate, chlorine-containing residues such as hygroscopic AlCl.sub.3 form on the features. The chlorine residues react with adsorped moisture from the atmosphere to form hydrochloric acid which corrodes the aluminum features. The corrosion problem is particularly severe for alloys such as Al-Cu alloy and Ti-W alloy, because galvanic coupling of the metals in these alloys results in corrosion of the alloys even at trace levels of residual chlorine and low levels of ambient moisture.
In passivating techniques, the etchant byproducts on the substrate are inactivated using a passivating gas. For example, a passivating method useful for passivating chlorine etchant byproducts on aluminum-containing features comprises exposing the etchant byproducts to a CF.sub.4 plasma. It is believed that some of the chlorine ions on the features are replaced by the fluorine ions in the CF.sub.4 plasma to form non-hydroscopic species, such as AlF.sub.3. Corrosion of the passivated substrate is reduced because the non-hydroscopic species does not corrode the features.
In corrosion inhibition techniques, a corrosion inhibiting deposit is formed on the surfaces of the metal-containing features. One inhibition method comprises exposing the substrate to a CHF.sub.3 plasma. It is believed that the CHF.sub.3 exposure causes a sidewall film to deposit on the features, thus "sealing" the surface of the features with a corrosion inhibiting layer that prevents ambient moisture from contacting the substrate.
After passivating and stripping, the sidewall deposits on the sidewalls of the features on the substrate are removed by applying a wet chemical etchant solution, such as an acidic solution, to the substrate. The chemical etchant solution dissolves the sidewall deposit on the substrate.
Conventional passivating and inhibition techniques have several limitations. One limitation is that the conventional techniques which have acceptable process throughput, can prevent post-etch corrosion of the substrate only for short periods, typically ranging from about 1-5 hours after exposure of the substrate to ambient moisture. The short corrosion resistant period is undesirable because it necessitates that post-etch processing steps be performed within 1-to-2 hours after the substrate is first exposed to the atmosphere, resulting in a tight and inflexible production schedule. Often, the substrate cannot be etched, or if already etched, cannot be removed from the passivating and stripping chamber, until the next processing station is available. This precludes efficient use of the etching and passivating equipment. Also, unexpected processing delays, such as that caused by equipment failures can cause the loss of an entire batch of wafers. Cumulative manufacturing losses can exceed tens of millions of dollars.
Even longer corrosion resistant periods, typically at least about one week, are desirable after wet chemical etching of the substrate. The extended corrosion resistance period is desirable so that the partially processed substrate can be stored while waiting for the next processing step. Conventional passivating processes are typically unable to provide these extended corrosion resistant periods.
The short corrosion resistant period provided by conventional processes, results from their inability to remove all of the etchant byproducts on the metal features. Trace amounts of residual etchant byproducts left on the substrate cause delayed post-etch corrosion of the substrate. Conventional passivating processes can provide increased substrate corrosion resistance, when they are performed on the substrate for an extended duration of time. However, the extended duration passivation processes are commercially unacceptable, because they provide reduced process throughput.
Thus, there is a need for a passivating process that effectively passivates the etchant byproducts on the substrate so that the substrate exhibits substantially no post-etch corrosion for an extended period of time. There is also a need for a stripping process that can strip substantially all of the polymeric remnant resist portions remaining on the substrate. It is also be highly desirable to have a passivating and/or stripping process that is faster than conventional processes and provides high process throughput. It is also desirable to use conventional processing equipment to passivate and strip the substrate.